Vought XF5U

F5U
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The sole completed XF5U-1
Type	Fighter
Manufacturer	Vought
Maiden flight	1942-11-23
Status	Cancelled 1947-03-17
Number built	2

Vought V-173

The Vought XF5U-1 Flying Flapjack was an experimental U.S. Navy fighter aircraft which was designed during World War II by Charles H. Zimmerman. It is one of the most unusual-looking aircraft ever designed, consisting of a flat, somewhat disk-shaped body (hence its name) serving as the lifting surface. It was powered by two piston engines buried in the body, driving propellers on shafts which protruded from the leading edge at the wingtips. The original prototype, designated the V-173 (Flying Pancake), was built of wood and canvas and featured a conventional, fully symmetrical aerofoil section (NACA 0015). The development version, the XF5U-1, was a larger aircraft with all-metal construction, and was almost five times heavier, with two 1,600 hp Pratt and Whitney R-2000 radial engines. The configuration was designed to create a low aspect ratio aircraft with a low takeoff and landing speed and high top speed. Because of its very low stall speed it could possibly have proven itself to be an excellent reconnaissance aircraft.

The first flight of the V-173 was on November 23, 1942. Flight testing of the V-173 would continue for over a year. During that time it would be flown successfully by Charles Lindbergh, who found it surprisingly easy to handle. On one occasion the V-173 was forced to make an emergency landing on a beach. As the pilot made his final approach he noticed two bathers directly in his path. The pilot locked the plane's brakes on landing causing the aircraft to flip over onto its back. Remarkably, the airframe proved so strong that neither the plane or the pilot sustained any significant damage.

The aircraft's most significant problem concerned its complicated gearbox, which permitted power from the engines to be transferred to its two long propeller shafts. In ground testing the gearbox produced unacceptable amounts of vibration, delaying the aircraft's first test flight for months. To make matters worse, the gearbox was constructed partially out of silver, making the aircraft unusually expensive.

The lone XF5U-1 had undergone ground runs, but the vibration problem was never overcome. It is believed that on one occasion the XF5U-1 actually became airborne for a short period, but the height and duration of the flight, if indeed it actually occurred, is unknown.

Though the design was promising, it came at the time where the United States Navy was switching from propeller driven planes to jet propelled planes. By 1946 the XF5U-1 project was already long over its expected development time, and well over budget. With jet aircraft coming into service the Navy finally cancelled the project on March 17, 1947 and the prototype aircraft (V-173) was transferred to the Smithsonian Museum for display. The only completed XF5U-1 proved to be so structurally solid that it had to be destroyed by a wrecking ball. The scrap was sold to a salvage company which would later become involved in a government investigation when it attempted to resell the recovered silver.

It is possible that with further development the XF5U-1 could have entered production, but it would have been a very complicated aircraft both to produce and maintain and like its sibling, the F7U Cutlass, would likely have had a short service life.

How it works

The XF5U looks like it should not be able to fly, as its wing area looks so small. Normally, a wing with such a low aspect ratio will suffer from very poor performance due to the degree of induced drag created at the wingtips, as the higher pressure air below spills around the wingtip to the lower-pressure region above. In a conventional aircraft, these wingtip vortices carry a lot of energy with them and hence create drag. The usual approach to reducing these vortices is to build a wing with a high aspect ratio—i.e. one that is long and narrow. However, such wings compromise the maneuverability and roll rate of the aircraft, or else will present a structural challenge in building them stiffly enough. The XF5U overcomes the tip vortex problem in a radical way, by using the propellers to actively cancel the drag-causing tip vortices. The propellers are arranged to rotate in the opposite direction to the tip vortices, which retains the higher-pressure air below the wing. Since this source of drag is eliminated, the aircraft will fly with a much smaller wing area, and the small wing will yield high maneuverability and present no structural difficulties.

An obvious problem with this arrangement on the XF5U-1 was that the propeller's radius covered nearly the entire frontal area of the aircraft. A typical installation of any forward-firing weapons such as machine guns, cannon, or missiles would be virtually impossible. Any machine guns or cannon were to be placed against the nose on either side of the cockpit. Also, the radar would have to be mounted forward of the propellers to prevent interference. (Similar problems were to be encountered in the design of the Bell/Boeing V-22 Osprey).

Specifications (XF5U-1)

General characteristics

• Crew: One, pilot
• Length: 28 ft 7 in (8.73 m)
• Wingspan: 32 ft 6 in (9.91 m)
• Height: 14 ft 9 in (4.50 m)
• Wing area: 475 ft² (44.2 m²)
• Empty weight: 13,107 lb (5,958 kg)
• Loaded weight: 16,722 lb (7,600 kg)
• Max takeoff weight: 18,772 lb (8,533 kg)
• Powerplant: 2× Pratt & Whitney R-2000-7 radial engine, 1,350 hp (1,007 kW each) each

Performance

• Maximum speed: 425 mph (775 km/h)
• Range: 1,064 miles (1,703 km)
• Service ceiling: 34,492 ft (10,516 m)
• Rate of climb: 718 ft/min (219 m/min)
• Wing loading: 35 lb/ft² (172 kg/m²)
• Power/mass: 0.16 hp/lb (0.27 kW/kg)

Armament

• 6 × .50 machine guns or
• 4 × 20 mm machine guns or
• 2 × 1000 lb. bombs

External link

• The XF5U story

Related content

Comparable aircraft

• V-22 Osprey

Designation sequence
F2U - F3U - F4U - XF5U - F6U - F7U - F8U Related lists

• List of fighter aircraft
• List of military aircraft of the United States